Having handled a few R51 pistols, and I’m distinctly unimpressed. Even the current ones are garbage.

The simple answer is that it uses a weird alternative operating mechanism to lock/unlock the breech, and that Browning’s tilting-barrel system is superior in virtually every way. Once the Browning patent(s) expired, there was no longer any reason to make pistols based on inferior alternatives, like the Pedersen system used in the R51

I believe another key issue with the R51 is the extensive use of MIM components — The design requires several parts in the slide to be very precisely sized, and to be able to smoothly slide against each other; Dimensional accuracy issues (which can be a problem with MIM parts, especially when QC is an issue) and surface tolerances can both lead to the sort of timing/reliability issues that have plagued the R51. MIM isn’t necessarily bad on it’s own, but in order to use that manufacturing technique the part really has to be designed to be made that way from the ground up.

Here’s my list: Mauser broom handle. the Luger, the 1911, any gun like a 1911 that needs a long pointy “belly stabber” beaver-tail, any gun that has a Glock style trigger safety as it’s only safety, any kind of revolver.